Or, for that matter, if Chamberlain hadn't pressured France into reneging on its treaty with Czechoslovakia by stating that the British would not support France if they were to go to war to support the Czechoslovaks.
Actually, your best bet is to take Neville Chamberlain (& his government) out of the picture entirely.
Note: I don't think Churchill would have been any better, he was vehemently opposed to the creation of the independent states of both Czechoslovakia & Poland after WW1, until WW2 commenced & they became politically convenient.
PS: Don't forget that Germany & the USSR had the Molotov–Ribbentrop Pact in August 1939, so the Soviets would have stayed neutral until they saw which way the battle was turning.
Well, Chamberlain was actually rearming as fast as he could. Of course the British were paralysed by "the Bomber Must Get Through Theory" and thought JU-88s would level London in an afternoon causing 35k casualties a day. Of course those horrid plebs would then surrender, where in fact it created what we all now know as 'Blitz mentality'. The Brits always were over-focused on the whole 'morale thing.
The USSR & Germany make the Molotov–Ribbentrop Treaty*
after France backs out of the Franco-Soviet Pact and it became apparent to Uncle Joe & Co. that the West led by Britain saw the coming Nazi-Bolshevik War as a very good thing. In fact Truman said on the campaign trail that if the Soviets started to win the USA should side with the Nazis, something that sort of stunk later. Stalin thought that signing the treaty would let him clear out the 'right tendency**' in the USSR and try and secure the Karelian Peninsula to safeguard Leningrad from the Finnish who the USSR saw as a Nazi vassal state. Of course we all know how well that went. Worse, it convinced the Nazi hierarchy and the Wehrmacht that the USSR was a pushover. It can be said that 'let the Nazis and the Bolsheviks slaughter each other' was Churchill's war strategy especially considering how he dicked around trying to make the Mediterranean a British lake until the Soviets threatened to make peace with the Nazis and let them come sort out his manoeuvring down there. Of course it was all for nought as the US put a stop to this Mediterranean silliness in 1956.
(*The Western Narrative portrays any treaty it doesn't like as a 'pact'. Funnily enough the EastBloc called NATO 'The North Atlantic Pact' :) and called their own alliance The Warsaw Treaty Organisation)
(**Although how you could get more 'rightish' than Stalin defies me)